To understand the molecular events coupling between cell proliferation and differentiation by elucidating genes essential for the process, we conducted a large scale gene expression analysis of an in vitro osteoclastogenesis system consisting of recombinant RANKL and mouse RAW264 cells. The entire process leading to the formation of tartrate resistant acid phosphatase-positive multinucleated cells takes 3 days and plates become fully covered with multinucleated cells at 4 days. Microarray probing at eight time points revealed 635 genes that showed greater than 2-fold differential expression for at least one time point and they could be classified into six groups by the "k-means" clustering analysis. Among a group of 106 early inducible genes (within 2-5 h after RANKL stimulation), four genes including NFAT2 were identified as genes whose enhanced expressions were fairly correlated with an efficient induction of matured osteoclasts. Moreover, cyclosporin A significantly suppressed the multinucleated cell formation accompanying the reduction of the nuclear localization of NFAT2. When the expression of NFAT2 was suppressed by introducing antisense NFAT2, multinucleated cell formation was severely hampered. Functional analysis thus combined with gene analysis by microarray technology elucidated a key role of NFAT2 in osteoclastogenesis in vitro.Specific factors/regulatory genes playing essential roles for cellular differentiation have been identified in various systems, and they have been shown to exert their effects eventually through the induction or repression of certain groups of genes (1-4). Therefore, gene expression profiling based on fine statistical analysis in addition to an elucidation of key factors/ genes might be essential to understand the molecular mechanisms underlying the differentiation process of a certain cell type. Fortunately, recent advances in the technology for assaying RNA in a highly parallel fashion (5-7), coupled with the completion/progress of several mammalian genome projects, make the approach feasible if a refined system is available. Here, we describe the broad outlines of gene expression during osteoclastogenesis in vitro, in particular during the initial stage, and explain the identification and characterization of genes essential for osteoclastogenesis on the basis of profiling characteristics. A similar approach using a different cell system was reported recently (8).Osteoclasts are multinucleated (MN) 1 giant cells and present only in bone with the capacity to resorb mineralized tissues (9). They were reported to be formed by fusion of mononuclear precursor cells derived from colony-forming unit granulocyte macrophages (CFU-GM) and branch from the monocyte-macrophage lineage during the early stage of the differentiation process (9, 10). Recently, a key factor responsible for initiating this differentiation process was identified and named receptor activator of NFB ligand (RANKL) (or osteoclast differentiation factor (ODF)/TNF-related activation-induced cytokine TRANCE) (11-13)...

Estrogen deficiency causes bone loss, which can be prevented by estrogen replacement therapy. Using a recently developed technique for isolation of highly purified mammalian osteoclasts, we showed that 17 β-estradiol (E2) was able to directly inhibit osteoclastic bone resorption. At concentrations effective for inhibiting bone resorption, E2 also directly induced osteoclast apoptosis in a dose- and time-dependent manner. ICI164,384 and tamoxifen, as pure and partial antagonists, respectively, completely or partially blocked the effect of E2 on both inhibition of osteoclastic bone resorption and induction of osteoclast apoptosis. These data suggest that the protective effects of estrogen against postmenopausal osteoporosis are mediated in part by the direct induction of apoptosis of the bone-resorbing osteoclasts by an estrogen receptor– mediated mechanism.

In bone development and regeneration, angiogenesis and bone/cartilage resorption are essential processes and are closely associated with each other, suggesting a common mediator for these two biological events. To address this interrelationship, we examined the effect of vascular endothelial growth factor (VEGF), the most critical growth factor for angiogenesis, on osteoclastic bone-resorbing activity in a culture of highly purified rabbit mature osteoclasts. VEGF caused a dose-and time-dependent increase in the area of bone resorption pits excavated by the isolated osteoclasts, partially by enhancing the survival of the cells. Two distinct VEGF receptors, KDR/Flk-1 and Flt-1, were detectable in osteoclasts at the gene and protein levels, and VEGF induced tyrosine phosphorylation of proteins in osteoclasts. Thus, osteoclastic function and angiogenesis are upregulated by a common mediator such as VEGF.z 2000 Federation of European Biochemical Societies.

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SummaryOsteopetrotic (op/op) mice have a severe deficiency of osteoclasts, monocytes, and peritoneal macrophages because of a defect in the production of functional macrophage colony-stimulating factor (M-CSF) resulting from a mutation within the M-CSF gene. In this study, we examined whether daily 5-,ug injections of purified recombinant human M-CSF (rhM-CSF) for 14 d would cure these deficiencies in the mutant mice. Monocytes in the peripheral blood of the oplop mice were significantly increased in number after subcutaneous injections of the factor two or three times a day. In contrast, osteopetrosis in the long bones of op/op mice was completely cured by only one injection of rhM-CSF per day. Bone trabeculae in the diaphyses were removed . Many osteoclasts were detected on the surface of bone trabeculae in the metaphyses . Although development of tooth germs ofuninjected oplop mice was impaired, rhM-CSF injection restored the development ofmolar tooth germs and led to tooth eruption as a consequence ofthe recovery ofbone-resorbing activity. These results demonstrate that M-CSF is one ofthe factors responsible for the differentiation of osteoclasts and monocyte/macrophages under physiological conditions.

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